A device which adds an SSB facility to a receiver not designed to receive such signals. It is associated with the receivers CDW and CDY, which, with the FAZ added, were used mainly as general purposes receivers in the monitoring of SSB Voice and F1 RATT signals for engineering purposes. Fitted widely throughout the Fleet. Generally speaking, the B40D catered for SSB voice and F1 RATT at a wide shift of 850Hz whereas the B41C was used only for F1 RATT at a narrow shift of 85Hz or 50Hz.